The Jacobite is the steam-hauled tourist train that runs from Fort William to Mallaig and back along the West Highland Line. The route — past Loch Eil, over the Glenfinnan Viaduct, around the head of Loch Eilt and along the white beaches of Arisaig — is often called the most scenic railway journey in Britain, and it is hard to argue.
Two services usually run in each direction in the high season, fewer in spring and autumn. Book months ahead if you want a seat, particularly for the morning train; standard-class compartments are cheaper and just as scenic as first. The regular ScotRail trains that use the same line all year round are dramatically cheaper and run on the same tracks; for the view, they will do.